Under what geological structures do the correct conditions for diamond formation exist?

The correct conditions for diamond formation primarily exist under cratons. Cratons are stable and ancient parts of the continental crust which have remained largely unchanged for billions of years. They provide the high-pressure and high-temperature environments necessary for the gradual formation of diamonds deep within the Earth's mantle.

Diamonds form at depths of about 150 to 250 kilometers (93 to 155 miles), where the pressure and temperature are sufficient to create carbon crystals. Cratons, being the most stable parts of the Earth's crust, often host the geological formations known as mantle xenoliths, which are the key indicators that diamonds might be found in these areas.

While volcanic activity can transport diamonds to the surface through kimberlite pipes found in cratons, the volcanic environment itself is not where diamonds are formed. Mountain ranges and ocean beds do not provide the necessary conditions for diamond creation, as they do not experience the extreme pressures and temperatures found in the mantle beneath cratons. Thus, cratons are uniquely positioned as the geological structures where diamonds form.